1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber. More particularly, it relates to a method for producing a glass preform for use in the fabrication of an optical fiber which contains fluorine.
2. Description of Related Art
A glass preform for use in the fabrication of an optical fiber is produced by various methods. Among them, the VAD method and OVPD method are attractive methods since the productivity and quality of the optical fiber fabricated are better than other methods. These methods comprise synthesizing glass soot particles by flame hydrolysis of a glass-forming raw material and depositing the soot particles on a rotating seed material to form a porous soot preform. Then, the porous soot preform is heated in a suitable atmosphere to dehydrate and vitrify it so as to produce a transparent glass preform, which is drawn to fabricate an optical fiber.
The optical fiber comprises a core through which light is propagated and a cladding which surrounds the core and reflects light to be propagated. The numerical aperture (hereinafter referred to as "N.A.") is calculated from the average refractive indexes n.sub.i and n.sub.2 of the core and the cladding as follows: ##EQU1## it is understood that the difference of the refractive index between the core and the cladding is increased to obtain large N.A. and one of following measures is taken in case of a silica (SiO.sub.2) glass type optical fiber:
1) In the core, an additive for increasing its refractive index is added.
2) In the cladding, an additive for lowering its refractive index is added.
3) A combination of the measures 1 and 2. Needless to say, the cladding in case 1 and the core in case 2 are made of silica glass.
Usually, GeO.sub.2, P.sub.2 O.sub.5, Al.sub.2 O.sub.3 and TiO.sub.2 are used to increase the refractive index of silica glass, and B.sub.2 O.sub.3 and fluorine are used to lower the refractive index of silica glass. FIG. 1 shows the variation of the refractive index of silica glass added with the various additives for light with a wavelength of 0.59 .mu.m (cf. Kumamaru and Kurosaki, "Light Transmission Materials" in the Industrial Materials (Kogyozairyo), 27 (1979) 39).
Among the additives, recently, the use of fluorine is the most attractive and it has been studied as an additive in the VAD method and the like.
To achieve the same difference of refractive index between the core and the cladding of the optical fiber, measure 2 or 3 is preferred since the additives are not added to the core, or only a smaller amount of the additive is added to the core than in the measure 1. This is advantageous for a high N.A. optical fiber since the attenuation of light transmission due to the presence of the additive is decreased. In addition, an optical fiber exhibiting good performance in the presence of radiation can only be produced by the measure 2. Therefore, it is greatly advantageous to add the additive to the cladding to lower its refractive index.
In the VAD method, fluorine is added in a sintering step, namely a vitrifying step of the soot preform. This has following advantages:
1. Fluorine is homogeneously added to achieve uniform distribution of the refractive index, and
2. An addition rate of fluorine is high. Namely, Several to ten hundreds grams of the porous soot preform can be treated and vitrified within several hours.
In the conventional methods, the soot preform is heated under a total pressure of one atmospheric pressure of an atmosphere of a gaseous fluorine-containing compound which is optionally diluted with an inert gas to add fluorine to the preform. However, according to the conventional method, undesirable bubbles tend to remain in the glass preform. Particularly in the VAD method, when fluorine is to be added in an amount corresponding to -0.5% of the refractive index difference, the bubbles remain in the glass. The amount of the bubbles increases as the added amount of fluorine increases.
Further, when the inert gas is used to dilute the fluorine-containing compound, any inert gas except helium, for example, nitrogen, argon or oxygen always forms bubbles in the glass preform. Helium sometimes forms bubbles in the glass preform. Although helium is a preferable diluting gas, it is expensive and increases the production cost of the optical fiber.
The reason why the bubbles are formed in the glass preform may be as follows:
When the soot preform is heated in the atmosphere comprising the fluorine-containing compound under a total pressure of one atmospheric pressure, gas trapped by the glass soot particles can not be removed and forms bubbles in the vitrified preform.
In case where the soot preform is heated in the inert gas such as nitrogen before fluorine is added, the inert gas may contained in pores in the soot particles. Therefore, if the soot preform is heated under the total pressure of atmospheric pressure, the inert gas cannot be dissipated from the surface of the soot preform so that a gaseous mixture of the inert gas and the fluorine-containing compound is present in the soot preform. Thus, the concentration of the fluorine-containing compound in the soot preform is lower than that in the atmosphere, particularly when a bulk density of the soot preform is 0.4 g/cm.sup.3 or larger. As the result, fluorine may not be added to the glass in a desired amount, or it may take longer to add a sufficient amount of fluorine to the glass.
The fluorine-containing compound penetrates in the soot preform through minute spaces in it and then the inert gas is exhausted from the preform. Therefore, replacement of the gas in the preform takes a much longer time.